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Questions regarding stability networks
Questions regarding stability networks
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Old 14th February 2018, 11:35 PM   #1
EL34Dave is offline EL34Dave  United States
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Default Questions regarding stability networks

Many amps have a series CR network (I believe itís also called a Zobel network) somewhere in the input stage, for the purpose of creating a HF roll off for stability. Some shunt the anode load of the input tube. Some, like the ST-70 (which Iím specifically interested in) incorporate this network between the input and the phase splitter, shunting to ground.

Iíve read that sometimes the value (pole) of this network is a little too aggressive, such that valuable HF in the audio range is impacted. So my questions are these.

What is the pole of the ST-70 network, 82pf/18k?

If necessary, how would I adjust the component values to raise the pole?

FWIW, I am just a total (and maybe a little obsessive) novice hobbyist, immersed in tweaking.

Thanks,
Dave
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Old 14th February 2018, 11:46 PM   #2
rayma is offline rayma  United States
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Originally Posted by EL34Dave View Post
What is the pole of the ST-70 network, 82pf/18k?
That RC is (AC wise) across the 270k plate resistor, so the 270k sets the stage's rolloff point
along with the 82pF, rather than the 18k. With a scope and generator, you can see how varying
the C affects the ringing. You may be able to reduce its value somewhat, perhaps by half,
with a given amplifier load before significant overshoot or ringing begins.

The input pentode also affects the rolloff point somewhat. The 18k should remain constant,
since it mainly smooths the phase response, by limiting the amount of hf gain reduction.

Be very careful, oscillations at high power will destroy loudspeakers. Use a dummy load.

Last edited by rayma; 15th February 2018 at 12:16 AM.
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Old 15th February 2018, 01:22 AM   #3
zigzagflux is online now zigzagflux  United States
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Whether you shunt the plate load (connecting plate to B+) or connect from plate to common 'ground', from an AC perspective they are equivalent. Makes no difference, though by habit I put it across the plate load.

A generalized rule of thumb (someone produced a fairly reasonable math proof which I read but cannot recall where it came from) is to size the resistance of the RC network to 10% of the value of the plate load. So with the ST-70 having a plate load of 270K, you could start out with a 27K resistor. From there, I tweak the desired capacitance value by scope with a square wave input. Produces very decent rise times with low overshoot. Sure, you can putz around with other R values, but 10% is a very good starting point. C makes all the difference on the scope.
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Old 15th February 2018, 01:34 AM   #4
nigelwright7557 is offline nigelwright7557  United Kingdom
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The formula is 1/(2 pi RC)=F

I usually use one on the first stage to stop HF at the input.
I remember building one of my first amps that didn't have one and it picked up local police radio very well !
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Old 15th February 2018, 01:38 AM   #5
rayma is offline rayma  United States
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Quote:
Originally Posted by nigelwright7557 View Post
The formula is 1/(2 pi RC)=F
This is called setting a dominate pole, or "slugging".
Usually it's necessary in an amplifier with an output transformer
and overall negative feedback.
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Old 15th February 2018, 01:54 AM   #6
trobbins is offline trobbins  Australia
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Shelf networks were becoming a well known stability aid back in the 1940's:

Corrective Networks for Feedback Circuits, V. Learned, Proc. IRE, July 1944.
http://dalmura.com.au/projects/corre...20networks.pdf

Stabilizing feedback amplifiers, T. Roddam, WW Mar 1951
http://www.americanradiohistory.com/...ld-1951-03.pdf
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Old 15th February 2018, 11:29 PM   #7
EL34Dave is offline EL34Dave  United States
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Quote:
Originally Posted by nigelwright7557 View Post
The formula is 1/(2 pi RC)=F

I usually use one on the first stage to stop HF at the input.
I remember building one of my first amps that didn't have one and it picked up local police radio very well !
The attached image shows the network I'm trying to figure out. Which resistor is used for the frequency calculation? 18K, results in a roll off frequency of 107883. 220K results in 8826. Significantly different. One is well out of the audio band, the other isn't.

Click the image to open in full size.
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Old 15th February 2018, 11:47 PM   #8
EL34Dave is offline EL34Dave  United States
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Quote:
Originally Posted by rayma View Post
That RC is (AC wise) across the 270k plate resistor, so the 270k sets the stage's rolloff point
along with the 82pF, rather than the 18k. With a scope and generator, you can see how varying
the C affects the ringing. You may be able to reduce its value somewhat, perhaps by half,
with a given amplifier load before significant overshoot or ringing begins.

The input pentode also affects the rolloff point somewhat. The 18k should remain constant,
since it mainly smooths the phase response, by limiting the amount of hf gain reduction.

Be very careful, oscillations at high power will destroy loudspeakers. Use a dummy load.
Hey Ray, I went back and read your response so you probably already answered my question. Please verify that the resistor which establishes the roll off frequency is the input tube's anode load, and not the one downstream of the cap.

Thanks,
Dave
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Old 15th February 2018, 11:54 PM   #9
rayma is offline rayma  United States
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Quote:
Originally Posted by EL34Dave View Post
Please verify that the resistor which establishes the roll off frequency is the
input tube's anode load, and not the one downstream of the cap.
Yes, approximately the input stage's roll off -3dB point is 1/(2Pi x 270k x 82pF) = 7.2kHz.
The input pentode's Rout also affects that frequency, but this formula is good enough to start.

Last edited by rayma; 15th February 2018 at 11:57 PM.
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Old 16th February 2018, 12:01 AM   #10
EL34Dave is offline EL34Dave  United States
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Quote:
Originally Posted by rayma View Post
Yes, approximately the input stage's roll off -3dB point is 1/(2Pi x 270k x 82pF) = 7.2kHz.
The input pentode's Rout also affects that frequency, but this formula is good enough to start.
Thanks.

The calculation with a 40pf cap works out to be about 18kHz, way more appropriate for HI FI, and probably still offering good HF stability.
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